1. Field of the Invention
This invention relates to electronic circuits used to generate timing signals and more specifically to circuits used to generate astable and monostable multivibrator such as '555 type timers.
2. Description of the Relevant Art
The problem addressed by this invention is encountered in circuits used for generating astable multivibrators. Astable multivibrators are commonly used, in combination with resistors and a capacitor to generate a square wave type signal. Such a square wave signal is used in many systems including computers, consumer electronic products, and automobiles.
A timer circuit that has a wide rage of astable and monostable possibilities is the low-power CMOS version of the 555 timer, manufactured as either an L555, XR555, or ICM7556. FIG. 1 shows the block diagram of an industry standard L555 as is known in the prior art. The circuit includes a comparator 2, a comparator 4, and a RS flip-flop 6. The RS flip-flop is typically constructed out of two nor gates, also shown in FIG. 1 as nor gates 8 and 10. FIG. 2 shows how the output Q is a function of the IN, IN+, and IN- inputs. These inputs can be controlled to determine the hysteresis of the circuit.
FIG. 3 shows in detail the prior art circuit typically used to implement the block diagram of the 555 timer circuit. Transistors 12-28 are configured as a typical comparator which corresponds to comparator 2 of FIG. 1. Similarly, transistors 30-46 correspond to comparator 4 of FIG. 1. Transistors 48-54 correspond to nor gate 8 and transistors 56-62 correspond to nor gate 10, both of FIG. 1. Resistor 64 and transistor 66 are necessary to provide the bias voltage to transistors 24 and 42. FIG. 3 also shows that 27 transistors are needed to use this circuit.
Therefore, it is an object of the invention to reduce the number of transistors used to make a timer circuit.
It is another object of the invention reduce the cost of a timer circuit by decreasing the number of transistors in the circuit.
It is and yet another object of the invention to increase the reliability of the circuit by decreasing the number of transistors in the circuit.
These and other objects, features, and advantages of the invention will be apparent to those skilled in the art from the following detailed description of the invention, when read with the drawings and appended claims.